I just started using an original headroom 'the max' (from www.headphone.com) with my sennheiser 580's. I don't know how it compares to the less expensive models, but I can't recommend one of these products enough. If you are happy with your current system, and optimal headphone performance is a concern, you might be well served by adding a headphone amp to your system. I know that grado makes a battery powered headphone amp that I see for sale in the 150-250 range here and there.
I have the max hooked up at work to my midfi phillips 880 cdr player. The headphone output of this thing is leagues better than any headphone output I've heard. It gives music a quietness, a purity of tone, and a sense of dynamic drive that I've never heard through headphones.
In the past I've listened to the following headphone outs with these same headphones. The $3000 sony was by far the best.
Sony xa7es cd player. (well behind the max w/phillips cdr880 !)
macintosh receiver 4275
higher end sony and panasonic portables w/battery.
phillips cdr880 (op amp blues)
denon 1025 receiver (hazy, but ok)
Denon 1520 (op amp blues, digititus)
phillips expanium mp3/cd player